The present invention relates to an exhaust gas purification unit for purifying the exhaust gas from an internal combustion engine which is preferably operated in lean burn mode.
A known exhaust gas purification unit is described in DE 40 32 085 A1. The corresponding exhaust gas purification installation includes an arrangement of denox or SCR (SCR=selective catalytic reduction) catalytic converters. These SCR catalytic converters catalyze a selective reduction reaction, in which nitrogen oxides are reduced under oxidizing conditions to form harmless nitrogen with the aid of ammonia which is introduced into the exhaust pipe or a reducing agent which releases ammonia. Ammonia is used as reducing agent, and additionally the upstream connection of a particulate filter for removing condensed exhaust gas constituents is proposed. These and similar installations remove the nitrogen oxide gaseous pollutants from oxygen containing exhaust gases, with the catalytic activity typically being effective in a temperature range between 200° C. and 450° C.
As statutory exhaust gas limit values are reduced further, effective lowering of the levels of other harmful exhaust gas constituents, such as for example carbon monoxide, hydrocarbons, has to be achieved in addition to the reduction in the levels of nitrogen oxides, in particular at relatively low exhaust gas temperatures.
An object of the present invention is to provide an exhaust gas purification unit which effectively removes pollutants within a wide temperature range.
This object has been achieved by an exhaust gas purification unit distinguished by a hydrogen generating unit. The hydrogen which is generated by this hydrogen generating unit is added to the exhaust gas unit on demand. What is known as a SCR catalytic converter is arranged in the exhaust gas purification unit downstream of a particulate filter. SCR catalytic converters often have a catalytic coating which is based on vanadium pentoxide and is almost exclusively matched to the selective reduction in the levels of nitrogen oxides under oxidizing conditions. However, SCR catalytic converters with a catalytic coating which contains precious metal(s) are also customary. Other reactions, such as for example oxidation reactions, are also catalyzed by SCR catalytic converters of this type. It is preferable for a corresponding SCR catalytic converter to be used in the exhaust gas purification unit according to the invention.
The oxidation of hydrogen is catalyzed particularly easily, i.e., even at relatively low temperatures. In this case, the oxidizing agents used may also be nitrogen oxides, which for their part are reduced to form nitrogen during the oxidation of the hydrogen. Therefore, the addition of hydrogen to the oxidizing exhaust gas leads to an effective reduction in the level of nitrogen oxides even at relatively low exhaust gas temperatures. At higher exhaust gas temperatures, the levels of nitrogen oxides can be reduced by selective reduction using the reducing agent ammonia at the SCR catalytic converter. It is thereby advantageously possible to reduce the levels of nitrogen oxides over a wide temperature range.
A further advantage of adding the gas which is generated by the hydrogen generating unit to the exhaust gas results from the more rapid heating of the corresponding catalytic converter or of the exhaust gas which is thereby achieved as a result of the exothermic oxidization of hydrogen, which is particularly easy to catalyze. This makes it possible, for example, to achieve accelerated heating of the SCR catalytic converter with regard to its SCR function.
Any desired device with which the person skilled in the art will be familiar may be used as the hydrogen generating unit. By way of example, a water electrolyzer or a methanol reformer may be suitable for applications in motor vehicles. The use of a separate hydrogen generating unit results in increased flexibility compared to methods in which a hydrogen containing gas is generated in the combustion chambers of the internal combustion engine by combustion technology measures. The increased flexibility consists firstly in the fact that the hydrogen containing gas can be used at times which are independent of engine operation, and secondly that if necessary it can be deliberately added to the exhaust pipe at different locations. The flexibility of the system can be increased still further if a temporary store is connected downstream of the hydrogen generating unit. The hydrogen containing gas mixture which is temporarily stored in this store can then be added to the exhaust gas in relatively large quantities at times when there is a high demand. The hydrogen generating unit can therefore also be made smaller than if a temporary store is not used.
In a currently preferred embodiment, the hydrogen generating unit is configured as a reformer, with the fuel which is in any case carried around for operation of the internal combustion engine being used. The reforming process in the reformer can be assisted in both purely thermal terms and catalytically by sub stoichiometric addition of air. The hydrogen content of the gas which is typically formed can be further increased by connecting a catalytic shift process downstream. In these shift processes which are known per se, carbon monoxide is reacted with steam to form carbon dioxide and hydrogen, with the result that the carbon monoxide content of the starting gas is correspondingly lowered and the hydrogen content is correspondingly increased.
In one configuration of the invention, the reducing agent supply has an ammonia generating unit for generating the ammonia which can be added to the exhaust gas from the internal combustion engine. This in particular avoids the need to carry around ammonia in gas or liquid form, which presents problems for motor vehicles. DE 199 09 933 A1 and DE 199 22 961, for example, have disclosed methods in which ammonia is provided by reduction of nitrogen oxide generated inside or outside the combustion source. Since this reduction takes place under reducing conditions in a catalytic converter integrated into the exhaust pipe, the internal combustion engine has to be operated under rich conditions at least from time to time, which presents difficulties in particular in the case of diesel engines.
By contrast, in a particularly advantageous configuration of the invention, the ammonia generating unit is configured as a nitrogen converting ammonia generating unit. In this context, the term nitrogen converting is to be understood as meaning that the ammonia generating unit uses elemental nitrogen to generate ammonia. The nitrogen which is used to generate ammonia can easily be removed from the ambient air or from the exhaust gas from the internal combustion engine. Therefore, the generation of ammonia is, on one hand, advantageously independent of the nitrogen oxides generated by the internal combustion engine or substantially independent of the internal combustion engine operating mode, and, on the other hand, the need for ammonia or ammonia releasing materials to be carried around onboard the motor vehicle is obviated.
In the exhaust gas purification unit according to the invention with nitrogen converting ammonia generating unit, this unit is, on one hand, supplied with the hydrogen containing gas from the hydrogen generating it and, on the other hand, with air or with an exhaust gas part stream removed from any desired location in the exhaust pipe. For this purpose, by way of example, in a first synthesis step nitrogen oxide is generated from the atmospheric nitrogen supplied or from the nitrogen in the oxygen containing exhaust gas supplied by a process which is known per se, such as a plasma process, or by an arc. In a preferably catalytically enhanced subsequent process, the nitrogen oxide generated in the first synthesis stage is then reduced to form ammonia when the hydrogen containing gas is added.
In a further advantageous configuration of the invention, a nitrogen oxide storage catalytic converter is arranged in the exhaust pipe upstream of the particulate filter. With this configuration of the invention, exhaust gas purification is achieved over a wide temperature range in particular by utilizing the temperature drop between the nitrogen oxide storage catalytic converter, particulate filter and SCR catalytic converter, given a suitable arrangement in the exhaust pipe. Specifically, if the temperature range at which the nitrogen oxide storage catalytic converter is effective is departed from in the event of a high engine load and therefore a rising exhaust gas temperature, the nitrogen oxide removal function can be carried out by the SCR catalytic converter which is located further downstream and is therefore at a lower temperature.
In another particularly advantageous embodiment of the exhaust gas purification unit according to the invention, an oxidization catalytic converter is arranged in the exhaust pipe upstream of the particulate filter. This arrangement too is advantageously suitable, with a view to generating a hydrogen containing gas or an ammonia containing gas, to allow exhaust gas purification within a very wide temperature range.
A further configuration of the invention is characterized in that the particulate filter is catalytically coated. This firstly facilitates the oxidization of oxidizable solid or non volatile constituents which have been deposited on the particulate filter. Secondly, the particulate filter coating can simultaneously catalyze the selective reduction of nitrogen oxide with hydrogen which takes place at low exhaust gas temperatures. Since the coated particulate filter has oxidation catalyzing properties, it is additionally advantageously possible for oxidizable and harmful constituents of gaseous nature to be removed from the exhaust gas at the same time as particulates are removed.
In a still further configuration of the invention, the hydrogen containing gas generated by the hydrogen generating unit can be added to the exhaust gas from the internal combustion engine on the entry side of the SCR catalytic converter and/or on the entry side of the particulate filter or on the entry side of the nitrogen oxide storage catalytic converter.
On account of the ready oxidizability of the hydrogen, the hydrogen containing gas is then advantageously added to the oxidizing exhaust gas on the entry side of the precious metal containing nitrogen oxide storage catalytic converter or SCR catalytic converter arranged in the exhaust pipe or on the entry side of the catalytically coated particulate filter if rapid heating of the corresponding component is desired. The heating results from the release of heat from the exothermic oxidation of the hydrogen at this component.
A further advantageous use of the hydrogen containing gas lies in the selective reduction of nitrogen oxides using hydrogen which takes place in particular at low exhaust gas temperatures (approx. 80° C. to 200° C.) at a precious metal containing catalytic converter or at the catalytically coated particulate filter. At low exhaust gas temperatures, by way of example, the levels of nitrogen oxides are reduced by the addition of the hydrogen containing gas on the entry side of the nitrogen oxide storage catalytic converter. The reduction of nitrogen oxides by hydrogen takes place at the catalytic centers of the precious metal containing nitrogen oxide storage catalytic converter. The addition of the readily oxidizable hydrogen containing gas moreover heats the nitrogen oxide storage catalytic converter by the exothermic oxidation process.
When the nitrogen oxide storage catalytic converter has reached its operating temperature for the removal of nitrogen oxides in alternating lean/rich mode, the addition of the hydrogen containing gas on the entry side of the nitrogen oxide storage catalytic converter can be terminated and the internal combustion engine can be switched over from pure lean burn mode to an alternating lean/rich mode. The levels of nitrogen oxides are then reduced in a known way by alternating storage of the nitrogen oxides in the nitrogen oxide storage catalytic converter when the internal combustion engine is in lean burn mode and reduction of the stored nitrogen oxides when the internal combustion engine is in rich burn mode. Overall, therefore, the levels of nitrogen oxides can be reduced by the nitrogen oxide storage catalytic converter within a wide temperature range from approx. 80° C. to approx. 400° C.
If necessary, the hydrogen containing gas can also be added on the entry side of the SCR catalytic converter and here too can be utilized at low temperatures for the selective reduction of the levels of nitrogen oxides and for the rapid heating to operating temperature. The optional addition of the hydrogen containing gas at the abovementioned locations therefore results in an optimum exhaust gas purification function of the overall exhaust gas purification unit.
In a yet further configuration of the invention, the hydrogen containing gas generated by the hydrogen generating unit can be added to the exhaust gas from the internal combustion engine on the entry side of the SCR catalytic converter and/or on the entry side of the oxidization catalytic converter. At low exhaust gas temperatures, the addition of the readily oxidizable hydrogen containing gas on the entry side of the oxidization catalytic converter allows the latter to be rapidly heated to its operating temperature for its oxidization catalyzing action. This makes it possible to achieve effective removal of oxidizable harmful constituents (carbon monoxide, hydrocarbons) from the exhaust gas very quickly as the engine is being warmed up, for example. At the same time, effective reduction in the levels of nitrogen oxides can be achieved by the selective reduction of nitrogen oxides which takes place at low temperatures at the precious metal containing oxidization catalytic converter.
The addition of the hydrogen containing gas generated by the hydrogen generating unit on the entry side of the SCR catalytic converter also advantageously enables the SCR catalytic converter to reach its operating temperature for the SCR function quickly. In the case of a precious metal containing SCR catalytic converter, the levels of nitrogen oxides can be reduced at this catalytic converter with the aid of the added hydrogen, preferably at low temperatures.